1. Field of the Invention
The present invention relates to a medical device according to the preamble of the independent claim.
2. Description of the Prior Art
Generally it is of major importance to be able to isolate people who are more susceptible to heart disease from people who are not in the danger zone.
Separating these patients early means that there should be ample time to suggest means of reducing the dangerous fat, thus decreasing the risk of heart disease and hospitalization. It is well known that the amount of visceral fat can be reduced by appropriate diets and exercise, which means that human suffering will be decreased and the costs for public health care will be lowered.
In fact, it is not how fat a person is that decides whether or not he or she is in danger of attaining a heart disease—it is the location of the fat. This means that not only obviously obese people or people with a high body mass index (BMI) are in danger, but many other people, especially women, that are not identified as in the risk zone as well. Recent research results have shown that it is not the subcutaneous fat, but instead the visceral fat that is of importance when judging whether or not somebody is in the risk zone of heart disease. Visceral fat is the fat surrounding the organs.
By using e.g. magnetic resonance imaging (MRI) it may be possible to get an idea of how large deposits of visceral fat a patient has. There are, however, alternatives to this method.
In the following, some exemplary impedance measurement systems are disclosed, using external electrodes applied on the skin of a patient, and used to determine the amount of fat tissue or visceral fat of a subject.
United States Patent Application Publication No. 2005/0107717 discloses a method and device for calculating visceral fat of a subject by a bioimpedance method applied by measuring the electric impedance ratio between the longitudinal width of the abdomen and the transversal width of the abdomen of the subject. The measurements are performed via electrodes applied on the skin of the subject.
U.S. Pat. No. 6,539,310 relates to a body type determination apparatus for providing an easy-understandable representation of fat tissue and lean tissue, respectively occupying in a body constitution of a subject. This is achieved by entering body data and from a measured bioelectric impedance also obtained by external electrodes.
Implantable impedance measurement systems are e.g. used in connection with heart stimulating devices to detect, e.g. oedema in a patient's lungs, or to monitor the condition of a patient's body tissue.
U.S. Pat. No. 5,282,840 relates to multiple frequency impedance measurement system for monitoring the condition of a patient's body tissue. The device includes electrodes for contacting the tissue to be monitored, circuitry for generating electrical signals of at least two frequencies for application to the tissue and circuitry for monitoring the impedance of the tissue, at the frequencies applied. The measured impedance values may be employed to detect changes in tissue condition, such as those induced by ischemia and by drug therapies. A microprocessor may average the stored impedance values over a period of hours, days, weeks, or even months, compare the measured impedances at the two frequencies, in order to identify changes of the impedance values for improving the stimulation therapy.
U.S. Pat. No. 6,473,640 relates to an implantable device and method for long-term detection and monitoring of congestive heart failure. A signal generator is employed to generate an electrical signal which is monitored to obtain a single or dual frequency measurement that can independently measure systemic venous and pulmonary (lung) impedance.
U.S. Pat. No. 5,876,353 discloses the use of an impedance monitor using direct current (DC) impedance measurements in order to obtain measurements of transthoracic impedance and respiratory rate of a patient. The measurements are taken over a long term to obtain a long term average signal so that differences can be used to assess the amount of tissue oedema over the long term changing condition.
Also United States Patent Application Publication No. 2005/028091 primarily relates to an edema detector, generally referred to as a method and apparatus for determining conditions of a biological tissue. The relative amount of intracellular and extracellular liquid (water) is determined by applying an excitation signal at two frequencies chosen that one of the frequencies may let the measurement current pass the cell membranes of the tissue. The resulting signals from the measurements are then processed, e.g. by cross-correlation calculations, and the result may be used to detect changes with regard to the amount of oedema.
The investigations concerning this field of medicine have determined, as discussed above, that visceral fat in general, regardless of its position, is more dangerous than subcutaneous fat. The present invention is based upon the fact that fat has much higher impedance than other parts of the body. Blood has for example a resistivity of approximately 1.6 Ωm, whereas fat has a resistivity of approximately 25 Ωm, which is about 15 times higher than blood. This is partly because blood contains 70-75% water, and fat (which is hydrophobic) contains as little as 10-20% water. Furthermore, it is worth to mention that the tissue in the body is mostly resistive with currents applied with a frequency below approximately 10 kHz. Other parts of the body have the following approximate values for the resistivity: heart: 2.5 Ωm (longitudinal) and 5.6 Ωm (transversal), which gives a weighted mean value of ˜4.6 Ωm; lungs 11-21 Ωm, ˜15 Ωm (mean), and other tissue approximately 10 Ωm.